X-linked lymphoproliferative disease (XLP) results in an inability of patients to mount normal immune responses to the Epstein-Barr virus (EBV). Invariably, infection with EBV results in fatal mononucleosis, agammaglobulinemia or B-cell lymphoma. Although XLP is a rare disease (1/1,000,000) we have adequate material (260 affected males and 186 carriers) available to carry out the isolation of the gene. The XLP gene lies within a 10 cM region in Xq25 between DXS42 and DXSIO. There are no candidate genes in the region. We seek to clone the XLP gene on the basis of its map position. We have identified interstitial deletions in four XLP patients (43-004 63-001, 94-02 and 92-13) involving an approximately 0.6-3.0 Mbp region in Xq25. The purpose of this study is to isolate cosmid and PI clones of the region specific YACs (915c7, 916d1 and 903f12) and to construct a cosmid/P1 contig for the deleted region of 0.6 Mbp in patient 92-13. Isolation of DNA sequences from the cosmid and PI subclones will provide a set of closely spaced physical markers, sequence tagged sites (STSs) and polymorphic DNA probes. They will be used to screen for polymorphism which could reduce the size of the XLP critical region and to look for linkage. They will be used to analyze DNA from XLP patients for chromosomal aberrations. The YAC-based mapping will allow us to identify candidate genes in the critical region based on exon trapping, direct selection for cDNAs, and searching for genes associated with CpG-enriched DNA fragments. The XLP gene will be selected from these candidate genes on the basis of its consistent mutation in XLP patients and its tissue-specific expression. Once the gene is identified we intend to determine its structure and function and to examine the tissue distribution of expression. Our large series of XLP patients will be studied to determine the type and frequency of different XLP mutations. Full-length cDNA will be constructed and sequenced and the amino acid sequence of the putative XLP protein will be deduced. Sequence comparison will be done with other proteins known to be active within the immune system. Although XLP is a rare disease, it severely effects those families who carry the mutated gene. Males in these families are at high risk of morbidity or mortality. By cloning the XLP gene we will be able to understand the interaction between EBV and the immune system and to develop further methods for detecting XLP mutations, including prenatal diagnosis of the disease.